A A spinal tap, also called a lumbar puncture, is a procedure performed to remove a small amount of cerebrospinal fluid—the fluid that bathes your brain and spinal cord—for laboratory analysis. It is used to check for bleeding around the brain (subarachnoid hemorrhage), infections such as meningitis or encephalitis, cancers, and other disorders of the brain, spinal cord, and nerves.

During the test, you will be asked to lie on your side in the fetal position, with your legs bent up, your neck bent down toward your chest, and your chin tucked. Sometimes this test is done in the sitting position, and you are then asked to lean forward, neck bent and arms on a tray table. These positions cause the vertebral spaces to open more and allow the needle to pass more easily.

A small area of the skin over the spine at about the level of the top of the pelvic bones is numbed with a local anesthetic (lidocaine). A very thin needle is inserted between the vertebrae to measure the pressure and remove the spinal fluid. There may be some brief, mild discomfort in your lower back after which the fluid is allowed to drip into vials. This procedure usually takes about 10 minutes. It's best to lie flat for a while afterwards to minimize the chances of a headache.